There is a need for being able to temporarily provide an individual car with reliable warning/protection equipment communicating by way of a radio connection with a nearby alarm central. Typically, this is of interest in a scenario in which a number of cars, for example, have arrived during the day, while in the same period perhaps roughly the same number of cars have been removed. The object then is to distribute and place an alarm device onto or into each individual car in a simple manner that minimizes the effort for an employee at the site. If each alarm unit needs to be positioned accurately and configured correctly for the alarm to function properly, a lot of time will be spent in carrying out this task, hence making the use of the alarm systems prohibitively time-consuming.
Alarm devices for the temporary protection of cars as indicated above are previously known, for example, from U.S. Pat. No. 3,864,675, U.S. Pat. No. 4,155,067, U.S. Pat. No. 4,187,497, and U.S. Pat. No. 5,463,371. In U.S. Pat. No. 3,864,675, an alarm device is disclosed that needs to be hanged inside the car in a vertical position and that responds to movements/vibration of the car body. U.S. Pat. No. 4,155,067, U.S. Pat. No. 4,187,497, and U.S. Pat. No. 5,463,371 also disclose alarm devices responding to movements/vibration, which are carefully positioned using a hanger means suspended from the top of one of the windows of the car.
Thus, it appears that the site personnel have to spend a lot of time positioning and adjusting the alarm device appropriately for each individual car so that it will be able to cooperate properly with the monitoring central. This is a consequence, inter alia, of the type of detection principle being used.
Ideally, an employee should only have to open a car door, then put or even toss the alarm device into the car seat without any concern for its position or orientation, and thereafter just close and lock the door. Alternatively, the employee could hide the alarm device under the seat or in the trunk, for example. The object of the present invention is to provide an alarm device that following such a simple deployment is still able to function 100% reliably.
Hence, an important part of the present invention is that the detection principle being used works omnidirectionally, meaning that the detector receives external signals from all directions. Another important part is to avoid blocking of the path for the external signal towards the detector, regardless of how the alarm device is positioned within the car.
Thus, according to the present invention, a portable alarm device for protecting a car against theft or burglary is provided, the alarm device comprising                a housing having at least one opening through at least one of the housing walls for providing an air flow path to a microphone located within the housing, the microphone being adapted for detecting infrasound signals within the frequency range of 0.1-15 Hz,        electrical circuitry within the housing for processing signals received from the microphone and transmitting an alarm signal on the automatic determination that a detected event is abnormal, and        a power supply embedded in the housing.        
In particular, the alarm device according to the invention is characterized in that the housing is provided with steering features for ensuring the existence of an unobstructed air flow path to the at least one opening regardless of the orientation assumed by the alarm device when put in by the employee to rest on one of the available support surfaces within the coupé of the car, e.g. a seat, floor, or trunk.
According to a first embodiment, said steering features are comprised of at least one projection or leg on the housing in the vicinity of the microphone opening.
According to an alternative embodiment, the steering features may consist of a netting or grating secured to the housing and spaced at a certain distance from the housing surrounding the opening. In this case, the netting may enclose the entire housing like a ball or an egg, and the netting/grating may be secured to the housing at the housing corners, or be secured to the housing by way of short struts. Preferably, the struts are flexible in order to provide a shock dampening function. Moreover, it would be advantageous if the netting/grating were split through a hinge and locking arrangement to provide easy access to the housing. The netting/grating is preferably made of a strong plastic material.
According to a further embodiment, the steering features may be comprised of a number of openings in the form of perforations in the housing wall(s) formed in the housing wall(s) in areas ensuring that at least one of the perforations points upwards/outwards regardless of the orientation of the alarm device. The perforations may advantageously be distributed throughout the entire outer surface of the housing.
In some embodiments it will be advantageous to have a soft outer casing surrounding the housing, preferably of a soft rubber material, for mechanical shock absorption.
Also, it is advantageous if the at least one opening in the housing wall is covered by a membrane preventing the intrusion of dust while allowing infrasound signals to pass.
In an embodiment wherein the electrical circuitry is connected to an acoustic alarm signaller, the alarm signal emitted may be an audio signal.
In another embodiment, the electrical circuitry is connected to a radio antenna for transmitting a radio signal to an external alarm central.